1. Field of the Invention
The present invention is directed generally to a method for detecting the presence of a watermark in cellulose-based material and, more particularly, to a method of applying a perfluorocarbon and/or hydrofluorocarbon compound to paper for temporarily revealing a watermark.
2. Description of the Background
Stamps and other documents made for special purposes, particularly official purposes, often have a watermark on or in cellulose-based material that is not clearly visible to the unaided eye. Generally, the watermark is made by mechanically pressing or forming the paper, or by applying a chemical additive in the paper manufacturing process. This indelible mark is added for identification, verification, or other official purposes, and prevents illegal activities such as, for example, tampering, counterfeiting or forgery.
Detection of the presence of a watermark is generally accomplished through the aid of chemical additives. Numerous patents are directed to invisible chemical watermarks that can be subsequently detected by the addition of a reagent to the cellulose material that will change the color of the watermark to make it visible.
For example, U.S. Pat. No. 5,139,57 to Kawashima discloses the use of a colorless ink containing at Least one color changing agent which changes to a visible color upon the application of a color former. The color changing agents can be pH indicators having a colorless pH range. Kawashima, however, discloses several complex agents that allow the watermarks to fade at different rates of speed.
U.S. Pat. No. 2,445,586 to Simons discloses a coating for paper or other articles that permanently changes color when moisture is applied. The coating is preferably a dye that has been decolorized by an alkaline earth hydroxide but wherein the color is restored when moistened.
U.S. Pat. No. 1,589,210 to Murray discloses an invisible reagent that is placed on or in paper which changes color when an ink eradicator is applied or mechanical erasure is employed. When an ink eradicator containing acid or a bleaching agent is subsequently applied to the paper a color is permanently developed. Where such paper is written or printed upon with acid ink and mechanical erasure is employed a color is again developed in the paper under the ink.
In addition, it is known to use carbon tetrachloride, benzene, hexane, lighter fluid and other reagents for revealing a watermark. However, use of these chemicals is undesirable due to the health concerns associated with their application. In addition, these chemicals tend to obscure the watermark, which appeers mottled when revealed.
Accordingly, the prior art patents disclose chemicals and their method of application that pose health concerns, ineffectively reveal the mark, dissolve ink or otherwise damage the document, are relatively complex, and/or permanently reveal the watermark. These drawbacks are particularly undesirable for revealing watermarks in collectibles such as stamps and the like.
Therefore, the need exists for a simple method of temporarily revealing a watermark at varying rates of speed, parlicularly useful for collectible documents such as stamps and the like, that clearly reveals the mark, does not damage the document, and is safe to use.